In third generation (3G) Universal Mobile Telecommunications Systems (UMTS) and in particular in its evolved version System Architecture Evolution/Long Term Evolution (SAE/LTE), also referred to as Evolved Packet System (EPS), the concept of home base stations is introduced. In 3G, a home base station is referred to as a Home Node B (HNB) whereas in EPS it is referred to as a Home eNodeB (HeNB). A cell served by a HNB or a HeNB is commonly referred to as a femtocell. A home base station is assumed to be placed in a private home, utilizing the home owner's fixed broadband connection to access the core network. It is also assumed that the home owner handles the actual physical installation of the home base station. Hence, the deployment of home base stations cannot be planned, since it is largely outside the control of the operator. Another important property of the home base station concept is the potentially very large number of home base stations.
In the further description a 3G Home Node B is abbreviated HNB, an EPS/LTE Home eNodeB is abbreviated HeNB and the abbreviation HN is used to denote either a 3G Home Node B or an EPS/LTE Home eNodeB.
A HN connects to the operator's network via a secure tunnel (supposedly IPsec protected) to a security gateway at the border of the operator's network. Via this tunnel the HN connects to the core network nodes of the operator's core network. The 3GPP operator may also deploy a concentrator node in its core network between the HNs and the regular core network nodes. (3GPP (3rd Generation Partnership Project) is a standardization body which specifies the standards for 3G/UMTS and EPS/SAE/LTE. A 3GPP operator is an operator which operates a telecommunication system implementing one or more of the 3GPP standards.) In the EPS standardization such a concentrator node is commonly referred to as a HeNB Gateway, which may be an optional node in EPS HeNB solutions. The corresponding node name in 3G UMTS standardization is HNB Gateway and this node is mandatory in 3G HNB systems. In this specification both HNB Gateways and HeNB Gateways may also be referred to as HN Gateways.
For both EPS and 3G UMTS the HN uses a broadband access network (e.g. Digital Subscriber Line technology (xDSL), Cable) as part of the transport network. Through this setup a mobile terminal, also known as user equipment (UE), communicates via the HN and the core network like any other UE.
The HN concept is closely related to the concept of Closed Subscriber Group (CSG). A femtocell is also assumed to be a CSG cell. This means that only a selected group of subscribers are allowed to access the network through that cell. The CSG of a CSG cell is identified by a CSG ID, which is broadcast in the cell as a part of the system information. Typically each CSG cell has its own unique CSG ID but it is also possible to define the same CSG ID for multiple cells, thereby forming a CSG zone, in which the same selected group of subscribers is allowed access. Although a CSG cell in principle does not have to be a femtocell, the two terms are often used as synonyms.
Hence, all subscribers are not allowed to access a certain HN and a certain subscriber is not allowed to access all HNs. Under supervision of the operator the owner of a HN (or the administrator of the CSG(s) associated with the cell(s) served by the HN) defines which subscribers are allowed to access a femtocell (CSG cell) of the HN (i.e. which subscribers that are included in the CSG of the femtocell). This is assumedly done through a web interface (or other “interface” between the HN owner and the operator), and the CSG data (or HN access list (which is an equivalent term assuming that the HN only serves one CSG)) is stored in a database in the operator's network. The HN owner would assumedly enter the allowed subscribers in the form of ISDN (Integrated Services Digital Network) numbers (MSISDN, Mobile Subscriber ISDN Number) or IMSIs (International Mobile Subscriber Identities) of the concerned subscribers.
The search for allowed CSG cells is not governed by the network, but is left to the UE to handle autonomously. To identify an allowed CSG cell the UE must read the CSG ID from the system information broadcast in the cell and compare it with the CSG ID(s) stored in a list referred to as CSG Whitelist or Allowed CSG List. When a match is found the UE has discovered an allowed CSG cell. To make the search for an allowed CSG cell more efficient the UE is assumed to utilize a so-called “fingerprint” of the CSG cell location. The exact mechanisms of such a fingerprint aided cell search are not standardized and will be up to each UE vendor to design. However, e.g. a fingerprint is obtained by listening to transmissions from the macrocells (i.e. regular cells which typically are much larger than femtocells) in the area of the allowed CSG cell. Transmissions from both LTE cells, 3G cells and 2G cells may be utilized to form a “signature”/“fingerprint” of the location of the CSG cell. Transmitted data that may be useful indications include e.g. cell identities (E-CGI (Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) Cell Global Identity) in EPS and CGI (Cell Global Identity) in 2G/3G), registration area identifiers, such as Tracking Area Identities (TATs) in EPS, Location Area Identities (LAIs) and Routing Area Identities (RAIs) in 2G/3G and UTRAN Registration Area (URA) identities in 3G. Other types of radio transmissions could potentially also be used to provide input data to the fingerprint, e.g. Wireless Local Area Network (WLAN) or Worldwide Interoperability for Microwave Access (WiMAX) transmissions. The UE learns the fingerprint (i.e. records relevant received data which can be used to identify the location) of a CSG cell when it is present in the cell. Later, when searching for the CSG cell (or before searching for the CSG cell) the UE scans the radio environment and when it receives an indication of a fingerprint match (i.e. sufficiently many of the pieces of data constituting the fingerprint match the received ones), the UE determines that it is worthwhile to check the CSG ID of detected potential CSG cells. Also the network may be aware of the fingerprint, e.g. the network may know in which macro tracking area (i.e. a tracking area comprising macrocells) a UE has an allowed CSG cell. The network can have learnt this when the UE accessed the network using a CSG cell.
When many femtocells are deployed in an area these will substantially contribute to the overall radio transmission interference and may degrade the quality of the radio environment in the macrocells as well as the general performance of the cellular system in the area. In particular this is a problem for a UE located close to a HN which it is not allowed to access.
Furthermore, a general trend in the telecommunication business is that, as a result of rising energy prices, the energy consumed by the network equipment represents an increasingly larger part of the cost of running the network. Together with the global environmental trend and the increasing importance for enterprises to have good environmental images, this has caused operators to demand that vendors of infrastructure equipment come up with ways to reduce their equipment's energy consumption.